Figures from the British Lung Foundation found that more than 430,000 children could be exposed to second-hand smoke in cars each week.

The law, which relates to England and Wales, was seen as a huge step forward in tackling this problem, applying to both drivers and passengers who smoke with under-18s, though not to those travelling alone or in convertibles with the roof down.

Scotland is to introduce a similar ban after MSPs voted in favour of a change to the law in December last year.

Responding to the FOI figures, the Department for Health said: “In changing the law we always said the measure of success would be in changes in attitude and behaviour, not number of enforcement actions.”